How much power does a belt driven fan use
 

I am not asking, I am about to find out.
I think this will be the most dangerous most insane series of tests to date.

I will take my speedy air compressor with 220 volt 2 horsepower 2 pole motor remove the belt and small pulley off and install the 5 inch fan adaptor pulley I am working on.

I have found that the belt driven fans on vehicles are usually slightly over driven. On my 3/4 ton suburban the fan and coolant pump are over driven by 25% of crank speed. At a not completely unreasonable crank speed of about 2700RPMs the fan would be spinning at the speed I want to test at, 3400RPM.

Instrumentation I have is:
A 220 volt kill-a-watt meter knock off. I have had it for a few years but have not tired it out yet. It should display true power, apparent power, give power factor in addition to volts and amps.
My 5kw 220v varrac.
Returning from my power steering power consumption tests; the optical tachometer.
The new addition, the flir.

I think I am going to test it something like this:

Just put the air compressor on the trailer and cargo strap it down, then just put the fan on the motor and go for it.
The 2 main tests I am concerned with are the fan "free wheeling" and a test with the viscous clutch locked up.

To test the fan free wheeling I would want to warm it up some but not enough to engage the clutch. As it would be while driving along.
Then engage the clutch. To engage the fan clutch just attach my heat gun to a stick aimed at the bi-metal strip and turn it on.
Test it for watts and RPMs with my optical tachometer on a long stick.
I am expecting the motor speed to drop down to 3400 to 3500 RPM.
I expect the motor to draw a lot of power, it will be plugged into my welder circuit that is on a 60 amp breaker.

Then take those RPM readings and use the varrac on the motor with no load and try to cut voltage down low enough to allow the motor magnetic field to slip at that same RPM to get a base line. I could almost just not do this test because I tested the motor when I got it and at no load its very efficient only drawing about 200 watts. I am expecting the motor with the fan attached to draw more like 20 to 30 amps.

I searched around and only found wildly differing numbers.
What looked like the most well documented numbers were provided by some one trying to sell you their fan, so they cant be trusted.
Or the numbers were obtained from "race engines" using solid flex fans, with no or few other details. Was the engine losing 10 horsepower to the fan at 9,900RPM or 6,000RPM, no one seems to know.
No one seems to have provided results of an economy test, with the fan free wheeling at highway speed.
Which is you know, only the most useful scenario by far to have data on.

Based on the first time I replaced a belt fan with electric fans my diesels fuel mileage went up 2mpg.
I believe it takes up to 40 horsepower to propel a suburban along at highway speed. If all of a sudden I am using 10% less fuel after I lose the belt driven fan, the engine is doing that much less work. So just free wheeling that large fan could be using up to 3 or 4 horsepower.
3 to 4 horsepower the whole time while you are driving from point A to B is huge.

Only if you get your head too close to the fan....

Two things to watch for: The fan power will be different with or without the shroud. Propeller type fans are very sensitive to back pressure, the power goes up as the airflow is restricted.

Very interesting. I'm subscribed.

The old aircooled type 1 VW fans were said to stall at 3800 rpm engine speed. I understand that pulling the belt was good for 20 hp for drag racing. Also suggested was that it could only cool 40hp worth of engine. Anything above 40hp would start heating things up quickly.

fan effect minutia
 

In 1974,when CAR and DRIVER aero-modded their Ford PINTO, the 4-blade,fixed-blade fan was removed and they gained 1-mpg @ 70-mph.
There were no electric fans in the day,and they needed some cooling,so they converted the fan to a fixed,2-blade unit and figured that they gained 1/2-mpg @ 70-mph.
Later,flex-fans came out which deformed centrifugally,which unloaded the engine at high speeds,where a fan was unnecessary.
Thermal,viscous-drive fans WERE in existence at the time,and theoretically,at speed,would be completely unloaded unless under strenuous circumstances.
With stringent CAFE standards now,carmakers are going to computer-controlled electric water pumps,split-flow cooling systems,electric cooling fans,and computer-controlled radiator blocks.
ISUZU has even eliminated the radiator with their 'GLIDER',Class-8 tractor.
I think that the days of the belt-driven fan are numbered.

The viscous clutch still spins the fan at quite a high rate of speed even when its unloaded.
I believe I read that a light duty viscous clutch spins the fan at 20 to 30% of pulley speed when unloaded and that a heavy duty clutch could spin the fan as much as 50% of pulley speed while unloaded.
Guess I will find out.

...replacing a belt driven fan with an electric one is possibly going to reduce engine warm-up times as well.

My Dodge/Cummins produces a furious wind when the clutch is disengaged and it's supposedly freewheeling. It might be part of the reason why it takes 10 minutes of driving before the engine warms up.

Subscribed if only so I can find out how much the roar my Tacoma makes when I start it is costing me. :eek:

If you have a Chevy v8, the Lincoln Mark VIII fan mod is sweet. Get it from a wrecker, the Volvo fan controller, the BMW 2 speed temp switch 90/99C and a relay with some wiring and you're done! I upgraded my water pump at the same time to an Edelbrock Victor series so I didn't have to figure where to put the temp switch. There was a spot on the new water pump. Keep the original shrowd and just use heavy duty tie straps to center/mount it!

I have pics on my web site if you check out silverado30mpg